Semiconductor device and method for fabricating the same

ABSTRACT

On an Si substrate  1,  a buffer layer  2,  a SiGe layer  3,  and an Si cap layer  4  are formed. A mask is formed on the substrate, and then the substrate is patterned. In this manner, a trench  7   a  is formed so as to reach the Si substrate  1  and have the side faces of the SiGe layer  3  exposed. Then, the surface of the trench  7   a  is subjected to heat treatment for one hour at 750° C. so that Ge contained in a surface portion of the SiGe layer  3  is evaporated. Thus, a Ge evaporated portion  8  having a lower Ge content than that of other part of the SiGe layer  3  is formed in part of the SiGe layer  3  exposed at part of the trench  7   a . Thereafter, the walls of the trench  7   a  are oxidized.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of International Patent ApplicationPCT/JP03/00141, filed Jan. 9, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device including alayer which contains Si and Ge and a method for fabricating the same,and more particularly relates to a semiconductor device which includes aSiGe layer or a SiGeC layer and is isolated from adjacent semiconductordevices by trench isolation and a method for fabricating the same.

In recent years, research and development has been actively carried outon heterojunction field effect transistors, heterojunction bipolartransistors, and like devices using an Si_(1−x)Ge_(x)(0<x<1) layer(which will be herein referred to as an “SiGe layer”) of a mixed crystalof silicon (Si) and germanium (Ge).

As for heterojunction devices using a SiGe layer, with increased carriermobility in the SiGe layer, high speed operation can be achieved. Inaddition, process steps for fabricating devices using a SiGe layer(which will be herein referred to as “SiGe devices”) can be usedinterchangeably with those for fabricating devices using an Si layer(which will be herein referred to as “Si devices”). Thus, fabricationtechniques and production lines for use in fabricating Si devices can beused in most of the process steps for fabricating SiGe devices.Therefore, it is also possible to integrate SiGe devices on an Sisubstrate. As can be seen from the above, SiGe devices exhibit excellentproperties in terms of performance and cost.

When a large number of SiGe devices are integrated on an Si substrate,isolation techniques for electrically insulating one device from anotherare critical as in the case of integrating known Si devices.

As known isolation techniques, local oxidation of silicon (LOCOS) andtrench isolation techniques have been used. When a LOCOS technique isused, however, there may be cases in which so-called birds' beaks areformed so that an oxide film for isolating devices from each otherenters into a transistor forming region. In such a case, the area of thetransistor forming region is reduced, and therefore it is difficult toavoid this situation and to achieve reduction in the size of integratedcircuits at the same time. Thus, trench isolation techniques which canisolate one device from another without causing reduction in the area ofthe transistor forming region are the mainstream isolation techniquesfor integrated circuits in accordance with the recent rules for sizereduction.

Hereinafter, isolation techniques for isolating devices including a SiGelayer will be described. In Japanese Unexamined Patent Publication No.10-321733 (U.S. Pat. No. 6,111,267), using a LOCOS technique, devicesincluding a SiGe layer are isolated from each other. However, for thereason described above, trench isolation techniques are now expected tobe the mainstream isolation techniques for isolating SiGe devices, as inthe case of isolating Si devices. Trench isolation techniques forforming a trench isolation can be roughly divided into two types. One isthe type in which a trench is formed before a SiGe layer is formed. Theother is the type in which a trench is formed after a SiGe layer hasbeen formed.

Hereinafter, a method for fabricating a semiconductor device in which atrench isolation is formed before forming a SiGe layer on a substratewill be first described. In this method, the process step of forming atrench isolation is performed before the process step of forming a SiGelayer, and thus process steps of a known method for fabricating an Sidevice can be used. This method, however, may cause some inconveniences.For example, assume that a SiGe layer is epitaxially grown on asubstrate in which a trench isolation has been formed. If the SiGe layerhas been non-selectively grown, a polycrystalline SiGe layer may beformed on an oxide film and/or a polycrystalline silicon film, resultingin the generation of leakage current. If the SiGe layer is selectivelygrown, facets may be created in the peripheral portion (boundaryportion) of part of the substrate in which the SiGe layer is selectivelygrown, thus resulting in fluctuation of the threshold voltage of adevice.

Next, a method for fabricating a semiconductor device in which a trenchisolation is formed after a SiGe layer has been formed on a substratewill be hereinafter described with reference to FIGS. 10A through 10E.FIGS. 10A through 10E are cross-sectional views illustrating respectiveprocess steps for forming a trench isolation in a substrate on which aSiGe layer has been formed in a known fabrication method. Note that inthe process steps shown in FIGS. 10A through 10E, a trench isolation isformed by the same process steps as those for forming a trench isolationin a known Si device.

First, in the process step shown in FIG. 10A, an Si buffer layer 102having a thickness of 10 nm, a SiGe layer 103 having a 25% Ge contentand a thickness of 15 nm and, an Si cap layer 104 having a thickness of15 nm are epitaxially grown by UHV-CVD on an Si substrate 101 containingan n-type impurity at a concentration of 1×10¹⁸ cm⁻³. In the UHV-CVD,Si₂H₆ (disilane) and GeH₄ (germane) are used as source gasses of Si andGe, respectively. The growth temperature is 550° C. and no intentionaldoping is performed. Note that the semiconductor substrate is dividedinto an active layer forming region Rac and an isolation region Rre forconvenience of description.

Next, in the process step shown in FIG. 10B, a silicon thermal oxidefilm 105 is formed by thermally oxidizing an upper portion of the Si caplayer 104. With this thermal oxidation, the upper portion of the Si caplayer 104 is oxidized to be a thermal oxide film, and therefore thethickness of the Si cap layer 104 becomes about 8 nm. Note that thethermal oxidation temperature is 750° C. Thereafter, a silicon nitridefilm 106 having a thickness of 210 nm is formed on the silicon thermaloxidation film 105. Note that the deposition temperature for the siliconnitride film 106 is 740° C. In this case, when each of the siliconthermal oxide film 105 and the silicon nitride film 106 is formed at alower temperature, it is possible to prevent defects from generating dueto relaxation of the strained SiGe layer 103 formed on an Si crystallinelayer.

Then, parts of the silicon nitride film 106 and the silicon thermaloxide film 105 located in the isolation region Rre are removed byanisotropic dry etching. Subsequently, the Si cap layer 104, the SiGelayer 103, the Si buffer layer 102 and an upper portion of the Sisubstrate 101 are patterned using as a mask remaining parts of thesilicon nitride film 106 and the silicon thermal oxide film 105 locatedin the active layer forming region Rac. In this manner, a trench 107 ais formed so as to have a depth of about 0.4 ì m to 0.8 ì m and reach tothe Si substrate 101. In this case, the side faces of the SiGe layer 103are exposed at the side surfaces of the trench 107 a by forming thetrench 107 a.

Next, in the process step shown in FIG. 10C, the surface of the trench107 a is thermally oxidized at 750° C., thereby forming a trench surfacecoating film 108 so as to coat the surface of the trench 107 a.

Next, in the process step shown in FIG. 10D, an oxide film is formed onthe substrate, and then part of the oxide film located in the activelayer forming region Rac is removed by etch-back or CMP (chemicalmechanical polish). In this manner, a trench oxide film 109 is formed soas to fill the trench 107 a. Thus, active regions are isolated from eachother by a trench isolation 107 including the trench oxide film 109 andthe trench surface coating film 108.

Next, in the process step shown in FIG. 10E, remaining parts of thesilicon nitride film 106 and the silicon thermal oxide film 105 locatedin the active layer forming region Rac in the substrate are removed byetching, so that part of the Si cap layer 104 located in the activelayer forming region Rac is exposed.

In connection with the fabrication of a semiconductor device including alayer containing Si and Ge, a process step for forming an oxide film bythermally oxidizing a surface portion of a trench in the above-describedmanner is disclosed in Japanese Unexamined Patent Publication No.10-74943 (U.S. Pat. No. 6,191,432). Furthermore, in the publication,disclosed is a semiconductor device fabrication method in which an Silayer having a thickness of about 5 nm to 50 nm is formed in a surfaceportion of a trench and then the Si layer is oxidized. This method isalso disclosed in Japanese Examined Patent Publication No. 6-80725 (U.S.Pat. No. 5,266,813 and U.S. Pat. No. 5,308,785).

Now, a semiconductor device including a trench isolation formed in theprocess steps shown in FIGS. 10A through 10E, i.e., a p-type MOSFET inwhich a SiGe layer serves as a hole channel (SiGe p-MOSFET) will bedescribed with reference to FIGS. 11A and 11B. FIGS. 11A and 11B arecross-sectional and plane views illustrating the structure of the p-typeMOSFET including a trench isolation formed in a known manner. FIG. 1A isa cross-sectional view taken along the line XI—XI shown in FIG. 11B.

An Si buffer layer 102, a SiGe layer 103, and an Si cap layer 104 areformed on part of an Si substrate 101 located in an active layer formingregion Rac. On the Si cap layer 104, a gate electrode 111 is formed witha gate insulating film 110 interposed between the Si cap layer 104 andthe gate electrode 111. Source/drain regions 112 heavily doped withp-type ions are provided in parts of the Si cap layer 104, the SiGelayer 103, the Si buffer layer 102, and the Si substrate 101 located onboth sides of the gate electrode 111 so as to be spaced apart from eachother. Then, parts of the Si cap layer 104, the SiGe layer 103, the Sibuffer layer 102, and the Si substrate 101 located under the gateelectrode 111, i.e., parts of the layers located between thesource/drain regions 112, serve as a channel region.

In an isolation region Rre of the Si substrate, a trench isolation 107including a trench oxide film 109 and a trench surface coating film 108coating the trench oxide film 109 is formed. With the trench isolation107, parts of the active layer forming region Rac which are to be activeregions are isolated from each other.

On the Si cap layer 104 and the trench isolation 107, an interlevelinsulating film 114 is formed so as to cover the gate electrode 111.Then, an interconnect 115 of, e.g., Al is formed so as to reach thesource/drain region 112 through the interlevel insulating film 114 andan SiO₂ film. A known p-MOSFET including a trench isolation has theabove-described structure.

As for the SiGe p-MOSFET of FIGS. 11A and 11B, however, when a trenchisolation is formed in the known process steps described in FIGS. 10Athrough 10E, the following inconveniences occur.

In the process step shown in FIG. 10C, thermal oxidation is performedwith the side faces of the SiGe layer 103 being exposed at the sidesurfaces of the trench 107 a. Thus, the trench surface coating film 108is formed. As the thermal oxidation process proceeds, Si is oxidized inthe side faces of the SiGe layer 103 exposed at the side surfaces of thetrench 107 a and also in part of the SiGe layer 103 located close to theexposed side faces. Thus, the part of the SiGe layer 103 in which Si hasbeen oxidized becomes parts of the trench surface coating film 108formed of SiO₂, and Ge is expelled from the trench surface coating film108. As a result, when the thermal oxidation process is finished, Gesegregates at the interface between the trench surface coating film 108and part of the SiGe layer 103 which has not been oxidized, therebyforming a layer containing Ge at a high concentration. Also, there havebeen reported cases in which depending on conditions for thermaloxidation, regions containing a high concentration of Ge are formed anddistributed like islands in the trench surface coating film 108.Moreover, Ge segregates in edge portions of the channel region locatedunder the gate electrode and at the interface between the active layerforming region Rac and the isolation region Rre, resulting in a Gesegregation layer 116, as shown in FIG. 11B.

Naturally, more interface states are generated at the interface betweenan SiO₂ layer and a SiGe layer than at the interface between an SiO₂layer and an Si layer. If many interface states are generated, thresholdvoltage in a device may fluctuate. Interface states may also form achannel for leakage current flowing between the source and the drain ina transistor or between different transistors.

The existence of part of the substrate which contain Ge at a highconcentration between the trench surface coating film 108 and the SiGelayer 103 may also cause fluctuation in threshold voltage.

FIG. 12 is a graph showing drain current-gate voltage characteristics inthe SiGe p-MOSFET shown in FIGS. 11A and 11B. The data shown in thegraph of FIG. 12 was obtained by measurements under the conditions inwhich both of the gate length and the gate width were 50 ì m and asource-drain voltage of −300 mV was applied. The graph shows thattransistor properties have been degenerated due to the above-describedinconveniences, such as an increase in leakage current and fluctuationin threshold voltage.

Ge segregation in performing thermal oxidation occurs not only in theprocess step of forming a trench but also in the process step of forminga gate oxide film on the SiGe layer. Then, in such a case, a gate oxidefilm has to be formed by oxidizing an Si cap layer with the SiGe layercovered by the Si cap layer.

Moreover, when an Si_(1−x−y)Ge_(x)C_(y) (0<x<1, 0≦y<1) layer (which willbe herein referred to as “a SiGeC layer”) is thermally oxidized, Gesegregation also occurs.

SUMMARY OF THE INVENTION

An object of the present invention is to devise means for suppressingthe occurrence of Ge segregation when a SiGe layer or a SiGeC layer isthermally oxidized and thereby to provide a semiconductor device inwhich leakage current can be suppressed and the threshold voltage hardlyfluctuates and a method for fabricating the same.

A method for fabricating a semiconductor device according to the presentinvention includes the steps of: a) forming a compound semiconductorlayer containing Si and Ge above a semiconductor substrate; b) removingpart of the compound semiconductor layer to form a trench; c) performingheat treatment on a surface portion of the trench; d) oxidizing at leastpart of the surface portion of the trench to form a thermal oxide film;and e) filling the trench with an insulator to form a trench isolationincluding the thermal oxide film and the insulator.

With the inventive method, Ge contained in part of a compoundsemiconductor layer located at a surface portion of a trench can beevaporated by heat treatment. Also, oxidation is performed after the Gecontent of the surface portion is reduced, and thus the amount of Gesegregation can be reduced. Accordingly, when a semiconductor device isin an operation state, the number of interface states to be generated atthe interface between a formed oxide film and the compound semiconductorlayer can be reduced. Therefore, it is possible to suppress leakagecurrent and fluctuation of threshold voltage in the device.

In the step c), the heat treatment may be performed under vacuum. Thus,Ge can be evaporated more effectively.

In the step c), the heat treatment may be performed in a non-oxygenatmosphere. Thus, Ge can be evaporated without oxidation beingprogressed in the compound semiconductor layer. Also, costs can bereduced.

The inventive method may further include after the step a), the step ofepitaxially growing an Si layer on the compound semiconductor layer.Thus, a strained Si layer can be formed, resulting in a semiconductordevice with a high current drivability.

If the step a) is characterized in that the compound semiconductordevice is formed on the semiconductor substrate with an insulating layerinterposed between the semiconductor substrate and the compoundsemiconductor layer, a semiconductor device can be electrically isolatedfrom adjacent devices with higher reliability.

The heat treatment may be performed at a temperature ranging from 700°C. to 1050° C. Thus, Ge can be evaporated without causing anyinconvenience due to degradation of the compound semiconductor layer orthe like.

In the step d), part of the compound semiconductor layer extending fromthe surface thereof located at the trench to a distance of 30 nm or lessmay be oxidized. Thus, a thermal oxide film can be formed of theoxidized part of the compound semiconductor layer which is not an activeregion.

In the step c), an upper portion of the compound semiconductor layer issubjected to heat treatment, in the step d), the upper portion of thecompound semiconductor layer is oxidized to form a gate oxide film, andthe method further includes after the step d), the step of forming agate electrode on the gate oxide film. Thus, the amount of Gesegregation between the gate oxide film and the compound semiconductorlayer can be reduced.

A semiconductor device according to the present invention includes: asemiconductor substrate; a compound semiconductor layer formed above thesubstrate and containing Si and Ge; an insulator; and a trench isolationincluding a thermal oxide film coating the insulator, wherein Ge iscontained in part of the compound semiconductor layer which is incontact with the thermal oxide film at a lower concentration than thatin another part of the compound semiconductor layer which is to be anactive region.

In the semiconductor device, the amount of Ge segregation in thevicinity of the interface between the thermal oxide film and thecompound semiconductor layer can be reduced. Accordingly, it is possibleto suppress leakage current and fluctuation of threshold voltage ofdevices when the devices are in an operation state, because thegeneration of interface states is suppressed.

In part of the compound semiconductor layer which is in contact with thethermal oxide film, at least some of Ge atoms are preferably evaporated.

An epitaxially grown Si layer may be provided on the compoundsemiconductor layer. Thus, the Si layer is strained and thereby thecurrent drivability of the device can be increased.

An insulating layer may be formed between the semiconductor substrateand the compound semiconductor layer. Thus, the semiconductor device canbe electrically isolated from adjacent devices with reliability.

The thermal oxide film may have a thickness of 30 nm or less. Thus, athermal oxide film can be formed of part of the compound semiconductorlayer which is not an active region.

If a gate oxide film and a gate electrode are further provided on thecompound semiconductor layer and the gate oxide film is formed byperforming heat treatment on an upper portion of the compoundsemiconductor layer to evaporate Ge and then oxidizing at least part ofthe upper portion, the amount of Ge segregation at the interface betweenthe gate oxide film and the compound semiconductor layer can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C are cross-sectional views illustrating respectiveprocess steps up to the process step of performing heat treatment in amethod for fabricating a p-MOSFET including a SiGe layer according to afirst embodiment of the present invention.

FIGS. 2A through 2C are cross-sectional views illustrating respectiveprocess steps up to the process step of forming a trench isolation inthe method for fabricating the p-MOSFET including a SiGe layer accordingto the first embodiment.

FIGS. 3A and 3B are cross-sectional views illustrating respectiveprocess steps subsequent to the process step of forming the trenchisolation in a method for fabricating a semiconductor device accordingto the first embodiment.

FIGS. 4A and 4B are cross-sectional and plane views illustrating astructure of the semiconductor device according to the first embodiment.

FIG. 5A is a graph showing results obtained by measuring using alow-speed ion scattering method the Ge content at the surface of a SiGelayer which has been subjected to heat treatment; and FIG. 5B is across-sectional view illustrating the measurement method.

FIG. 6 is a graph showing drain current-gate voltage characteristics forthe SiGe p-MOSFET shown in FIGS. 4A and 4B.

FIGS. 7A through 7C are cross-sectional views illustrating respectiveprocess steps up to the process step of performing heat treatment in amethod for fabricating a p-MOSFET including a SiGeC layer according to asecond embodiment of the present invention.

FIGS. 8A through 8C are cross-sectional views illustrating respectiveprocess steps up to the process step of forming a trench isolation inthe method for fabricating the p-MOSFET including a SiGeC layeraccording to the second embodiment.

FIGS. 9A and 9B are cross-sectional views illustrating respectiveprocess steps up to the process step of forming a gate oxide film in themethod for fabricating the p-MOSFET including a SiGeC layer according tothe second embodiment.

FIGS. 10A through 10E are cross-sectional views illustrating the processsteps of forming a trench isolation in a substrate on which a SiGe layerhas been formed in a known manner.

FIGS. 11A and 11B are cross-sectional and plane views illustrating astructure of a p-type MOSFET including a trench isolation formed in theknown manner.

FIG. 12 is a graph showing drain current-gate voltage characteristics inthe SiGe p-MOSFET shown in FIGS. 11A and 11B.

FIGS. 13A and 13B are cross-sectional views illustrating the processsteps of performing heat treatment and thermal oxidation, respectively,for a surface portion of a trench in a method for fabricating asemiconductor device according to a third embodiment of the presentinvention.

FIGS. 14A through 14D are cross-sectional views illustrating respectiveprocess steps up to the process step of performing thermal oxidation fora surface portion of a trench in a method for fabricating asemiconductor device according to a fourth embodiment.

FIG. 15 is a cross-sectional view illustrating an exemplary substrateused in the fourth embodiment.

FIG. 16 is a cross-sectional view illustrating another exemplarysubstrate used in the fourth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

In this embodiment, a method for fabricating a semiconductor deviceincluding a SiGe layer will be described with reference to FIGS. 1through 6. Note that a semiconductor device including an Si layer and aSiGe layer (i.e., a strained SiGe layer) grown on the Si layer will bedescribed as a semiconductor device including a SiGe layer.

First, process steps up to the process step of forming a trenchisolation in a method for fabricating a semiconductor device will bedescribed with reference to FIGS. 1A through 1C and FIGS. 2A through 2C.FIGS. 1A through 1C and FIGS. 2A through 2C are cross-sectional viewsillustrating respective process steps up to the process step of forminga trench isolation in the method for fabricating a p-MOSFET including aSiGe layer.

In the process step shown in FIG. 1A, using UHV-CVD, an Si buffer layer2 having a thickness of 10 nm, a SiGe layer 3 having a thickness of 15nm and a 25% Ge content and, an Si cap layer 4 having a thickness of 15nm are epitaxially grown on an Si substrate containing an n-typeimpurity at a concentration of 1×10¹⁸ cm⁻³. The UHV-CVD is performedusing Si₂H₆ (disilane) and GeH₄ (germane) as source gasses of Si and Ge,respectively. The growth temperature is 550° C. and no intentionaldoping is performed. Note that the semiconductor substrate is dividedinto an active layer forming region Rac and an isolation region Rre forconvenience of description.

Next, as shown in FIG. 1B, an upper portion of the Si cap layer 4 isoxidized at a temperature of 750° C., thereby forming a silicon oxidefilm 5 having a thickness of 15 nm. In this case, the silicon oxide film5 is formed by oxidizing part of the Si cap layer 4 and thus thethickness of the Si cap layer 4 itself is about 8 nm. Subsequently, asilicon nitride film 6 having a thickness of 210 nm is formed on thesilicon oxide film 5 at a temperature of 740° C. In this case, when eachof the silicon oxide film 5 and the silicon nitride film 6 is formed ata low temperature, it is possible to prevent defects from generating dueto relaxation of the strained SiGe layer 3 formed on Si crystals.

Then, parts of silicon nitride film 6 and the silicon oxide film 5located in the isolation region Rre are removed by anisotropic dryetching. Subsequently, the Si cap layer 4, the SiGe layer 3, the Sibuffer layer 2 and an upper portion of the Si substrate 1 are patternedusing as a mask remaining part of the silicon nitride film 6 located inthe active layer forming region Rac. In this manner, a trench 7 a isformed in part of the Si substrate 1 so as to have a depth of about 0.4ì m to 0.8 ì m. In this case, side faces of the SiGe layer 3 are exposedat the side surfaces of the trench 7 a by forming the trench 7 a.

Next, in the process step shown in FIG. 1C, heat treatment is performedat 750° C. for 60 minutes under a vacuum of 2.66×10⁻⁷ Pa. With the heattreatment, Ge is evaporated at part of a surface portion of the trench 7a at which the SiGe layer 3 is exposed. Herein, a surface portion of atrench means part of a substrate located in the vicinity of the surfacesof the trench but is not an active region of a device. With the Geevaporation, a Ge evaporated portion 8 is formed in part of the SiGelayer 3 exposed to the trench 7 a. The width R of the Ge evaporatedportion 8 is preferably as small as a width that does not affect deviceoperation. Specifically, when the width R is 30 nm or less, the Geevaporated portion hardly affects device operation.

In the SiGe layer 3, the part other than the Ge evaporated portion 8 isthe major portion having the same composition as that before the heattreatment. In the Ge evaporated portion 8, most of Ge contained beforethe heat treatment has been evaporated, and thus almost no Ge iscontained. However, when conditions for heat treatment are changed andonly part of Ge contained in the Ge evaporated portion 8 before the heattreatment is evaporated, Ge may be contained in the Ge evaporatedportion 8 at a lower content than that in the part of the SiGe layer 3other than the Ge evaporated portion 8.

When the heat treatment described above is performed at a temperatureranging from 700° C. to 1050° C., Ge can be evaporated without causingany inconvenience due to degeneration of the SiGe layer 3. In this case,if heat treatment is performed at a high temperature, time required forthe heat treatment is reduced. On the other hand, if heat treatment isperformed at a low temperature, the SiGe layer 3 can be kept more stableduring the heat treatment. In view of these points, the temperaturerange of heat treatment S is preferably from 700° C. to 950° C., andmore preferably from 750° C. to 850° C.

Time for heat treatment is preferably 120 minutes at a temperature of700° C., 90 minutes at a temperature of 750° C., 30 minutes at atemperature of 850° C., and 5 seconds at a temperature of 1050° C. Whenheat treatment is performed at some other temperature, time for heattreatment is set at time according to the temperature.

Heat treatment for evaporating Ge is performed under a vacuum of 133 Paor less, or a non-oxygen atmosphere. When heat treatment is performedunder a vacuum, Ge is evaporated more effectively with decreasingpressure. On the other hand, when heat treatment is performed in anon-oxygen atmosphere, pressure is maintained normal or reduced. In thelatter case, there is no need to keep a vacuum state and thus costs canbe advantageously reduced. Note that a non-oxygen atmospherespecifically means herein an atmosphere which contains an inert gas,nitrogen, or the like.

Next, as shown in FIG. 2A, the surface portion of the trench 7 a isthermally oxidized at 750° C., thereby forming a trench surface coatingfilm (thermal oxide film) 9 so as to coat the surface of the trench 7 a.In this case, part of the Ge evaporated portion 8 having a low Gecontent is oxidized to become part of the trench surface coating film 9in the SiGe layer 3. Thus, the amount of Ge segregation can be reduced,compared to the known method, and therefore interface states at theinterface between the trench surface coating film 9 and the SiGe layer 3can be reduced. At this time, part of the Ge evaporated portion 8located close to the trench 7 a is oxidized to be part of the trenchsurface coating film 9. Optionally, almost entire part of the Geevaporated portion 8 and part of the SiGe layer 3 located close to theGe evaporated portion 8 may be oxidized to be part of the trench surfacecoating film 9. This is because even in such a case, the amount of Gesegregation can be reduced to a lower level than that in the knownmethod. Moreover, the trench surface coating film 9 has preferably athickness of 30 nm or less, i.e., a thickness that does not affectdevice operation.

Next, in the process step shown in FIG. 2B, an oxide film for fillingthe trench 7 a is formed on the substrate and then part of the oxidefilm located in the active layer forming region Rac is removed byetch-back or CMP (chemical mechanical polish). Thus, a trench oxidelayer (insulator) 10 is formed to fill the trench 7 a. In this manner,the active layer forming regions Rac in the semiconductor substrate isisolated from each other with the trench isolation 7 including thetrench oxide layer 10 and the trench surface coating film 9 coating thetrench oxide layer 10.

Next, in the process step shown in FIG. 2C, parts of the silicon nitridefilm 6 and the silicon oxide film 5 located in the active layer formingregion Rac are removed by etching so that part of the Si cap layer 4located in the active layer forming region Rac is exposed.

Next, process steps subsequent to the process step of forming a trenchisolation in a method for fabricating a semiconductor device accordingto this embodiment will be described with reference to FIGS. 3A, 3B, 4Aand 4B. FIGS. 3A and 3B are cross-sectional views illustratingrespective process steps subsequent to the process step of forming atrench isolation in the method for fabricating a semiconductor deviceaccording to this embodiment. FIGS. 4A and 4B are cross-sectional andplane views illustrating the structure of the semiconductor device ofthis embodiment. Note that FIG. 4A is a cross-sectional view taken alongthe line IV—IV shown in FIG. 4B.

First, in the process step shown in FIG. 3A, an upper portion of theexposed part of the Si cap layer 4 located in the active layer formingregion Rac is thermally oxidized at 750° C., thereby forming a thermaloxide film 11 a having a thickness of 8 nm.

Next, in the process step shown in FIG. 3B, a polycrystalline siliconlayer having a thickness of about 200 nm is deposited on the thermaloxide film 11 a, and then B (boron) is ion-implanted into thepolycrystalline silicon layer. Thereafter, the polycrystalline siliconlayer and the thermal oxide film 11 a are patterned to form a gateelectrode 12 and a gate oxide film 11. Then, B (boron) is ion-implantedinto the substrate using as a mask the gate electrode 12 and the gateoxide film 11, thereby forming source/drain regions 13.

Thereafter, the following process steps will be performed to obtain thestructure shown in FIGS. 4A and 4B. An interlevel insulating film 14formed of silicon oxide and having a thickness of 500 nm is formed.Then, heat treatment is performed to activate impurities or the like inthe source/drain regions 13. Next, contact holes are formed through theinterlevel insulating film 14 so as to reach the corresponding one ofthe source/drain regions 13. Thereafter, each of the contact hole isfilled up to form an Al interconnect 15, so that the Al interconnect 15extends onto part of the interlevel insulating film. In the processsteps described above, the semiconductor device of this embodiment canbe obtained.

Now, Ge evaporation caused by the heat treatment described above will bedescribed with reference to FIGS. 5A and 5B. FIG. 5A is a graph showingresults obtained by measuring using a low-speed ion scattering methodthe Ge content at the surface of a SiGe layer which has been subjectedto heat treatment. FIG. 5B is a cross-sectional view illustrating themeasurement method.

The measurement method using a low-speed ion scattering will bedescribed. First, a SiGe layer having a 15% Ge content is grown on an Sisubstrate of the (001) plane orientation by UHV-CVD. Then, thissubstrate is introduced into a low-speed ion scattering analysisapparatus including a substrate heating system. With the substrate keptat a certain temperature for measurement, the surface of the substrateis bombarded with helium ions and then the time of flight of scatteringhelium ions is measured. Assume that the rates of heating and coolingthe substrate are set at 20° C./min and the temperature range formeasurement is set at a range from room temperature to 750° C. Thesubstrate is bombarded with helium ions at 3 keV.

In FIG. 5A, each of spectra (I) through (IX) indicates the spectrum ofthe flight time (TOF spectrum) of helium ions at each measurementtemperature. As shown in FIG. 5B, when the surface of the substrate isbombarded with helium ions (He⁺), helium ions are collided with surfaceatoms (mass M). Some of the collided helium ions (mass m) are scatteredat 180 degree to the incident direction. In this case, the flight timetaken for helium ions to reach a detector after an impact of the heliumions to the surface atoms is proportional to (M+m)/(M−m). Accordingly,by measuring spectra of flight times (TOF spectra), an element containedin the surface of the substrate can be specified.

As shown in FIG. 5A, TOF spectrum (I) for the substrate at roomtemperature (before subjected to heat treatment) has peaks at around6400 nsec and 5800 nsec representing the Ge and Si contents in thesubstrate, respectively. Spectra (II) through (IV) also have peaksrepresenting the Si and Ge contents in the substrate in the same manneras the spectrum (I). But, as for spectra (V) and (VI), the respectiveintensities of peaks representing the Ge content of the substrategradually decrease. This indicates that Ge evaporation has started to beobserved at around 700° C. Furthermore, as for spectra (VII) through(IX), the intensity of the peak representing the Ge content of thesubstrate decreases as well. But the spectrum (IX) has almost no peakFrom FIG. 5A, it can be understood that a large portion of Ge havingexisted around the surface of the SiGe layer are evaporated byperforming heat treatment on the substrate at 750° C. for 60 minutes.

The results described above shows that: when the substrate is heated to700° C. or more, Ge evaporation around the surface of the SiGe layeroccurs; and when time for heat treatment is increased, the amount ofevaporated Ge is increased. Therefore, it is sufficient that the heattreatment for Ge evaporation is performed at 700° C. or more. And timefor heat treatment may be changed according to the temperature at whichheat treatment is performed.

It has been also confirmed by the Ge content profiles of the SiGe layerin the inward direction that Ge is evaporated limitedly in a portion ofthe SiGe layer extending inward from the surface thereof by a distanceof about 15 nm and the composition of SiGe is not changed in a furtherinward portion of the SiGe layer. Therefore, if an oxide film having athickness of 30 nm or less is formed by oxidizing a portion of the SiGelayer located more inward than the portion extending inward from thesurface thereof by a distance of about 15 nm, effects of the presentinvention can be fully achieved.

Hereinafter, effects of this embodiment will be described.

First, after the Ge evaporation potion 8 has been formed by evaporatingGe in the part of the SiGe layer 3 exposed to the trench 7 a in theprocess step shown in FIG. 1C, part of the trench surface coating film 9is formed by oxidizing part of the Ge evaporated portion 8 in theprocess step shown in FIG. 2A. Thus, in the process step shown in FIG.2A, SiO₂ as good as one obtained by oxidizing Si can be obtained byoxidizing the Ge evaporated portion 8 with a low Ge content.

In addition, the Ge evaporated portion 8 of the SiGe layer 3 having alower Ge content than that in the rest of the SiGe layer 3 is oxidizedin this embodiment, and thus the amount of Ge segregation can bereduced. Therefore, a portion with a high Ge content is hardly formedaround the interface between the SiGe layer 3 and the trench surfacecoating film 9. As a result, a reduced number of interface states aregenerated. More specifically, the density of interface states generatedbetween the SiGe layer 3 and the trench surface coating film 9 is10⁹-10¹¹ cm⁻², which is the almost same value as that for the density ofinterface states generated at the interface between SiO₂ formed byoxidizing an Si layer and Si. As has been described, the segregation ofGe and the generation of interface states can be suppressed. Thus, it ispossible to suppress the occurrence of leakage current in part of theinterface between the SiGe layer 3 and the trench surface coating film 9located under the gate electrode 12. Accordingly, threshold voltagehardly fluctuates in a device.

FIG. 6 is a graph showing drain current-gate voltage characteristics forthe SiGe p-MOSFET shown in FIGS. 4A and 4B. Data shown in the graph ofFIG. 6 was obtained under the condition where both of the gate lengthand the gate width of the SiGe p-MOSFET were 50 ì m and a source-drainvoltage of −300 mV was applied. In FIG. 6, there is no hump on the curverepresenting subthreshold characteristics of threshold voltage in thesemiconductor device according to this embodiment. This shows that draincurrent in the OFF state is also sufficiently suppressed.

Although the semiconductor device in which a SiGe layer is used has beendescribed as an example in this embodiment, the present invention isapplicable to a semiconductor device including a layer containing Si andGe. Thus, a SiGeC layer may be used instead of a SiGe layer. As for thecomposition of SiGeC layers, for example, Ge content is 15% and Ccontent is 1%.

Moreover, this embodiment is applicable to a semiconductor deviceincluding a SiGe layer or a SiGeC layer formed on an SOI substrate.

(Second Embodiment)

In this embodiment, a modified example of the method for fabricating asemiconductor device according to the first embodiment will bedescribed. Hereinafter, description will be made using a semiconductordevice including a SiGeC layer as an example.

Process steps up to the process step of forming a gate insulating filmin a method for fabricating a semiconductor device according to thisembodiment will be described with reference to FIGS. 7A through 7C,FIGS. 8A through 8C and FIGS. 9A and 9B. FIGS. 7A through 7C, FIGS. 8Athrough 8C and FIGS. 9A and 9B are cross-sectional views illustratingrespective process steps up to the process step of forming a gateinsulating film in the method for fabricating a p-MOSFET including aSiGeC layer according to this embodiment.

First, in the process step shown in FIG. 7A, an Si buffer layer 22having a thickness of 10 nm, a SiGeC layer 23 having a thickness of 15nm, a 25% Ge content and a 0.7% C content are epitaxially grown byUHV-CVD on an Si substrate 21 containing an n-type impurity at aconcentration of 1×10¹⁸ cm⁻³. The UHV-CVD is performed using Si₂H₆(disilane), GeH₄ (germane) and SiH₃CH₃ as source gases of Si, Ge and C,respectively. In this case, the growth temperature is 500° C. and nointentional doping is performed. Note that the semiconductor substrateis divided into an active layer forming region Rac and an isolationregion Rre for convenience of description.

Thereafter, a deposited oxide film 24 is formed on the SiGeC layer 23 byCVD.

Next, in the process step shown in FIG. 7B, a silicon nitride film 25having a thickness of 210 nm is formed on the deposited oxide film 24.Then, parts of the silicon nitride film 25 and the deposited oxide film24 located in the isolation region Rre are removed. Using as a mask theremaining parts of the silicon nitride film 25 and the deposited oxidefilm 24 located in the active layer forming region Rac, the SiGeC layer23, the Si buffer layer 22 and an upper portion of the Si substrate 21are patterned, thereby forming a trench 26 a with a depth of about 0.4 ìm to 0.8 ì m.

Next, in the process step shown in FIG. 7C, the surface of the substrateis subjected to heat treament at 750° C. for 60 minutes under a vacuumof 2.66×10⁻⁷ Pa. With the heat treatment, Ge is evaporated in part of asurface portion of the trench 26 a at which the SiGeC layer 23 isexposed, thereby forming a Ge evaporated portion 28 in a portion of theS SiGeC layer 23 which extends inward from the surface thereof exposedto the trench 26 a by a distance of about 15 nm.

The width of the Ge evaporated portion 28 extending inward from itssurface is preferably as small as a width that does not affect deviceoperation. Therefore, if the width of the Ge evaporated portion 28extending inward from its surface is 30 nm or less, reduction in the Gecontent in part of an active region in a device or other inconveniencescan be avoided.

In the Ge evaporated portion 28, most of Ge contained before the heattreatment has been evaporated, and thus almost no Ge is contained.However, when conditions for heat treatment are changed and only part ofGe that has been contained in the Ge evaporated portion 28 before theheat treatment is evaporated, Ge may be contained in the Ge evaporatedportion 28 at a lower content than that in the part of the SiGeC layer23 other than the Ge evaporated portion 28.

When the heat treatment described above is performed at a temperatureranging from 700° C. to 1050° C., Ge can be evaporated without causingany inconvenience due to degeneration of the SiGeC layer 23. In thiscase, if heat treatment is performed at a high temperature, timerequired for the heat treatment is reduced. On the other hand, if heattreatment is performed at a low temperature, the SiGeC layer 23 can bekept more stable during the heat treatment. In view of these points, thetemperature range of heat treatment is preferably from 700° C. to 950°C., and more preferably from 750° C. to 850° C.

Time for heat treatment is preferably 120 minutes at a temperature of700° C., 60 minutes at a temperature of 750° C., 30 minutes at atemperature of 850° C., and 5 seconds at a temperature of 1050° C. Whenheat treatment is performed at some other temperature, time for the heattreatment is set at time according to the temperature.

Heat treatment for evaporating Ge is performed under a vacuum of 133 Paor less, or a non-oxygen atmosphere. When heat treatment is performedunder a vacuum, Ge is evaporated more effectively with decreasingpressure. On the other hand, when heat treatment is performed in anon-oxygen atmosphere, pressure is maintained normal or reduced. In thelatter case, there is no need to keep a vacuum state and thus costs canbe advantageously reduced. Note that a non-oxygen atmospherespecifically means herein an atmosphere which contains an inert gas,nitrogen, or the like.

In the process step shown in FIG. 8A, the surface of the substrate isthermally oxidized at 750° C., thereby forming a thermal oxide film 29on the surface of the trench 7 a. In this case, the Ge evaporatedportion 28 having a low Ge content is oxidized in the SiGeC layer 23.Thus, the amount of Ge segregation can be reduced, compared to the knownmethod, and interface states at the interface between the SiGeC layer 23and the thermal oxide film 29 can be reduced.

Next, in the process step shown in FIG. 8B, an oxide film for fillingthe trench 26 a is formed on the substrate and then part of the oxidefilm located in the active layer forming region Rac is removed byetch-back or CMP (chemical mechanical polish). Thus, a trench oxidelayer 30 is formed to fill the trench 26 a. In this manner, the activelayer forming regions Rac in the semiconductor substrate is isolatedfrom each other with the trench isolation 26 including the trench oxidelayer 30 and the trench surface coating film 29 coating the trench oxidelayer 30.

Next, in the process step shown in FIG. 8C, parts of the silicon nitridefilm 25 and the deposited oxide film 24 located in the active layerforming region Rac are removed by etching so that part of the SiGeClayer 23 located in the active layer forming region Rac is exposed.

Next, in the process step shown in FIG. 9A, the surface of the substrateis subjected to heat treatment at 750° C. for 60 minutes under a vacuumof 2.66×10⁻⁷ Pa. With the heat treatment, Ge is evaporated in an upperportion of the SiGeC layer 23. Herein, the upper portion of the SiGeClayer 23 means a region of the substrate located in the vicinity of thesurfaces of the trench but is not an active region. With the Geevaporation, a Ge evaporated portion 31 is formed in part of the SiGeClayer 23 which extends downward from the surface thereof by a distanceof about 15 nm.

In the SiGeC layer 23, the part other than the Ge evaporated portions 28and 31 is the major portion having the same composition as that beforethe heat treatment. In the Ge evaporated portion 31, most of Gecontained before the heat treatment has been evaporated, and thus almostno Ge is contained. However, there may be cases where only part of Gecontained in the Ge evaporated portion 31 before the heat treatment isevaporated due to heat treatment condition change or the like. In such acase, Ge may be contained in the Ge evaporated portion 31 at a lowercontent than that in the part of the SiGeC layer 23 other than the Geevaporated portion 31.

The conditions including temperature range, heating time, and atmospherefor the heat treatment in this process step are the same as those in theprocess step of FIG. 7C.

In the process step shown in FIG. 9B, the surface of the substrate isthermally oxidized at 750° C., thereby forming a gate oxide film 32 onthe SiGeC layer 23. In this case, the Ge evaporated portion 31 having alow Ge content is oxidized and thus the amount of Ge segregation can bereduced. Therefore, interface states at the interface between the SiGeClayer 23 and the gate oxide film 32 can be reduced.

Subsequent process steps are the same as those of the first embodiment.

In this embodiment, the same effects as those of the first embodimentcan be achieved. In addition to the effects, the following effects canbe also attained.

In this embodiment, the gate oxide film 32 is formed by oxidizing theupper portion of the Ge evaporated portion 31. Thus, unlike the knownmethod, there is no need to form a cap layer on the SiGeC layer.Therefore, the process steps can be simplified.

Note that in this embodiment, heat treatment for forming the Geevaporated portion 31 may be performed in any process step between theprocess step of forming the SiGeC layer 23 and the process step offorming the gate oxide film 32. Thus, the same effects can be attained.

Moreover, this embodiment is also applicable to a p-MOSFET using a SiGelayer.

Moreover, this embodiment is also applicable to a semiconductor deviceincluding a SiGe layer or a SiGeC layer formed on an SOI substrate.

(Third Embodiment)

In this embodiment, a semiconductor device including a SiGe layer, an Silayer (strained Si layer) grown on the SiGe layer will be described withreference to FIGS. 13A and 13B. FIGS. 13A and 13B are cross-sectionalviews illustrating the process steps of performing heat treatment andthermal oxidation for a surface portion of a trench, respectively, in amethod for fabricating a semiconductor device according to a thirdembodiment of the present invention.

To obtain such a substrate as one shown in FIG. 13A, the followingprocess steps are performed. First, a semiconductor substrate includingon a silicon substrate 41, a SiGe layer 42 having a 20-50% Ge contentand a thickness of 2.5 ì m, a relaxed SiGe layer 43 having a 50% or moreGe content and a thickness of 500 nm, a strained Si layer 44 epitaxiallygrown on the relaxed SiGe layer 43 and having a thickness of 50 nm, asilicon oxide film 45 having a thickness of 15 nm, and a silicon nitridefilm 46 having a thickness of 210 nm is prepared.

Then, parts of the silicon nitride film 46, the silicon oxide film 45,the strained Si layer 44 and part of the upper portion of the relaxedSiGe layer 43 located in the isolation region Rre are removed byanisotropic dry etching to form a trench 47 a through the siliconnitride film 46, the silicon oxide film 45, and the strained Si layer44.

Next, in the process step shown in FIG. 13A, heat treatment is performedat 750° C. for 60 minutes under a vacuum of 2.66×10⁻⁷ Pa. With the heattreatment, Ge is evaporated in part of the relaxed SiGe layer 43 locatedaround part of the surface of the trench 47 a at which part of therelaxed SiGe layer 43 is exposed. With the Ge evaporation, a Geevaporated portion 48 is formed in part of the relaxed SiGe layer 43extending inward from the surface thereof exposed to the trench 47 a bya distance of about 15 nm. Here, in the relaxed SiGe layer 43 the partother than the Ge evaporated portion 48 is the major portion having thesame composition as that before the heat treatment. In the Ge evaporatedportion 48, most of Ge contained before the heat treatment has beenevaporated, and thus almost no Ge is contained. However, there may becases where only part of Ge contained in the Ge evaporated portion 48before the heat treatment is evaporated due to heat treatment conditionchange or the like. In such a case, Ge may be contained in the Geevaporated portion 48 at a lower content than that in the part of therelaxed SiGe layer 43 other than the Ge evaporated portion 48.

When the heat treatment described above is performed at a temperatureranging from 700° C. to 1050° C., Ge can be evaporated without causingany inconvenience due to degeneration of the strained Si layer 44 or thelike. In this case, if heat treatment is performed at a hightemperature, time required for the heat treatment is reduced. On theother hand, if heat treatment is performed at a low temperature, thestrained Si layer 44 can be kept more stable during the heat treatment.In view of these points, the temperature range of heat treatment ispreferably from 700° C. to 950° C., and more preferably from 750° C. to850° C.

Time for heat treatment is preferably 120 minutes at a temperature of700° C., 90 minutes at a temperature of 750° C., 30 minutes at atemperature of 850° C., and 5 seconds at a temperature of 1050° C. Whenheat treatment is performed at some other temperature, time for the heattreatment is set at time according to the temperature.

Heat treatment for evaporating Ge is performed under a vacuum of 133 Paor less, or a non-oxygen atmosphere. When heat treatment is performedunder a vacuum, Ge is evaporated more effectively with decreasingpressure. On the other hand, when heat treatment is performed in anon-oxygen atmosphere, pressure is maintained normal or reduced. In thelatter case, there is no need to keep a vacuum state and thus costs canbe advantageously reduced. Note that a non-oxygen atmospherespecifically means herein an atmosphere which contains an inert gas,nitrogen, or the like.

Next, in the process step shown in FIG. 13B, a surface portion of thetrench 47 a is thermally oxidized, thereby forming a trench surfacecoating film 49 so as to coat the surface of the trench 47 a. In thiscase, part of the Ge evaporated portion 48 having a low Ge content isoxidized to become part of the trench surface coating film 49 in therelaxed SiGe layer 43, and thus the amount of Ge segregation can bereduced compared to the known method. Thus, interface states at theinterface between the trench surface coating film 49 and the relaxedSiGe layer 43 can be reduced. At this time, part of the Ge evaporatedportion 48 located close to the surface of the trench 47 a is oxidizedto be part of the trench surface coating film 49. Optionally, the almostentire part of the Ge evaporated portion 48 and part of the relaxed SiGelayer 43 located close to the Ge evaporated portion 48 may be oxidizedto be part of the trench surface coating film 49. This is because evenin such a case, the amount of Ge segregation can be reduced to a lowerlevel than that in the known method.

Thereafter, the trench 47 a is filled with a silicon oxide film to forma trench isolation. Furthermore, a device such as an MISFET is formed inpart of the substrate located in the active layer forming region Rae.The process step of forming a trench isolation and then forming a deviceis the same as that of the first embodiment. Therefore, description ofthe process step will be omitted.

In this embodiment, instead of the substrate described above, asubstrate with the layer structure including a strained Si layer thathas been disclosed in U.S. Pat. No. 5,534,713 (Japanese PatentPublication No. 2994227) may be formed. U.S. Pat. No. 5,534,713 ishereby incorporated by reference.

(Fourth Embodiment)

In this embodiment, a semiconductor device which includes a strained Silayer and has a different structure from that of the third embodimentwill be described with reference to FIGS. 14A through 14D. FIGS. 14Athrough 14D are cross-sectional views illustrating respective processsteps up to the process step of performing thermal oxidation for asurface portion of a trench in a method for fabricating a semiconductordevice according to a fourth embodiment.

First, in the process step shown in FIG. 14A, a SiGe layer (not shown)having a 30% Ge content and a thickness of 100 nm is bonded onto asubstrate including a silicon substrate 51 and a silicon oxide layer 52.Thereafter, heat treatment is performed at 800° C. for one hour, so thatthe SiGe layer is relaxed to become a relaxed SiGe layer 53.Subsequently, an strained Si layer 54 having a thickness of 50 nm isepitaxially grown on the relaxed SiGe layer 53.

Then, an upper portion of the strained Si layer 54 is oxidized at 750°C., thereby forming a silicon oxide film 55 having a thickness of 15 nm.On the silicon oxide film 55, a silicon nitride film 56 having athickness of 210 nm is formed at 740° C.

In the process step shown in FIG. 14B, a trench 57 a is formed byanisotropic dry etching in part of the substrate located in an isolationregion Rre so as to reach the silicon oxide layer 52 through the siliconnitride film 56, the silicon oxide film 55, the strained Si layer 54 andthe relaxed SiGe layer 53.

Next, in the process step shown in FIG. 14C, heat treatment is performedat 750° C. for 60 minutes under a vacuum of 2.66×10⁻⁷ Pa. With the heattreatment, Ge is evaporated around part of the surface of the trench 57a at which the relaxed SiGe layer 53 is exposed. With the Geevaporation, a Ge evaporated portion 58 is formed in part of the relaxedSiGe layer 53 extending inward from the surface thereof exposed to thetrench 57 a by a distance of about 15 nm. Here, in the relaxed SiGelayer 53, the part other than the Ge evaporated portion 58 is the majorportion having the same composition as that before the heat treatment.In the Ge evaporated portion 58, most of Ge contained before the heattreatment has been evaporated, and thus almost no Ge is contained.However, there may be cases where only part of Ge contained in the Geevaporated portion 58 before the heat treatment is evaporated due toheat treatment condition change or the like. In such a case, Ge may becontained in the Ge evaporated portion 58 at a lower content than thatin part of the relaxed SiGe layer 53 other than the Ge evaporatedportion 58.

When the heat treatment described above is performed at a temperatureranging from 700° C. to 1050° C., Ge can be evaporated without causingany inconvenience due to degeneration of the strained Si layer 54 or thelike. In this case, if heat treatment is performed at a hightemperature, time required for the heat treatment is reduced. On theother hand, if heat treatment is performed at a low temperature, thestrained Si layer 54 can be kept more stable during the heat treatment.In view of these points, the temperature range of heat treatment ispreferably from 700° C. to 950° C., and more preferably from 750° C. to850° C.

Time for heat treatment is preferably 120 minutes at a temperature of700° C., 90 minutes at a temperature of 750° C., 30 minutes at atemperature of 850° C., and 5 seconds at a temperature of 1050° C. Whenheat treatment is performed at some other temperature, time for the heattreatment is set at time according to the temperature.

Heat treatment for evaporating Ge is performed under a vacuum of 133 Paor less, or a non-oxygen atmosphere. When heat treatment is performedunder a vacuum, Ge is evaporated more effectively with decreasingpressure. On the other hand, when heat treatment is performed in anon-oxygen atmosphere, pressure is maintained normal or reduced. In thelatter case, there is no need to keep a vacuum state and thus costs canbe advantageously reduced. Note that a non-oxygen atmospherespecifically means herein an atmosphere which contains an inert gas,nitrogen, or the like.

Next, in the process step shown in FIG. 14D, a surface portion of thetrench 57 a is thermally oxidized at 750° C., thereby forming a trenchsurface coating film 59 so as to coat the surface of the trench 57 a. Inthis case, part of the Ge evaporated portion 58 having a low Ge contentis oxidized to become part of the trench surface coating film 59 in therelaxed SiGe layer 53, and thus the amount of Ge segregation can bereduced, compared to the known method. Thus, interface states at theinterface between the trench surface coating film 59 and the relaxedSiGe layer 53 can be reduced. At this time, part of the Ge evaporatedportion 58 located close to the surface of the trench 57 a is oxidizedto be part of the trench surface coating film 59. Optionally, the almostentire part of the Ge evaporated portion 58 and part of the relaxed SiGelayer 53 located close to the Ge evaporated portion 58 may be oxidizedto be part of the trench surface coating film 59. This is because evenin such a case, the amount of Ge segregation can be reduced to a lowerlevel than that in the known method.

Thereafter, the trench 57 a is filled with a silicon oxide film to forma trench isolation. Furthermore, a device such as an MISFET is formed inpart of the substrate located in the active layer forming region Rac.The process steps of filling a trench to form a trench isolation andthen forming a device are the same as those of the first embodiment.Therefore, descriptions of the process steps will be omitted.

In this embodiment, the substrate in which a SiGe layer is bonded ontothe silicon oxide film 52 is used. However, a substrate in which an Silayer is interposed between the silicon oxide film 52 and the SiGe layermay be used in the present invention. The structure of such a substratewill be described with reference to FIG. 15. FIG. 15 is across-sectional view illustrating an exemplary structure for a substrateused in the fourth embodiment.

As shown in FIG. 15, a substrate 70 includes a silicon substrate 61, asilicon oxide layer 62, an Si layer 63, a relaxed SiGe layer 64, astrained Si layer 65, a silicon oxide film 66, and a silicon nitridefilm 67. The Si layer 63 is formed on the silicon oxide layer 62 and hasa thickness of 50 nm. On the Si layer 63, the relaxed SiGe layer 64having a 30% Ge content and a thickness of 100 nm, the strained Si layer65 having a thickness of 50 nm, the silicon oxide film 66 having athickness of 15 nm, and the silicon nitride film 67 having a thicknessof 210 nm are formed.

As the process step of forming a substrate 70, a SiGe layer (not shown)having a 30% Ge content is formed on an SOI substrate including asilicon substrate 61, a silicon oxide layer 62 and an Si layer 63. Then,by performing the same heat treatment as that shown in FIG. 14A, theSiGe layer is relaxed to form a relaxed SiGe layer 64. Other processsteps are the same as those described in FIG. 14A.

Also, an Si layer and a Ge layer may be interposed between the siliconoxide film 52 and the SiGe layer 53 shown in FIG. 14A. This structurewill be described with reference to FIG. 16. FIG. 16 is across-sectional view illustrating another exemplary structure for asubstrate used in the fourth embodiment.

As shown in FIG. 16, a substrate 80 includes a silicon substrate 71, asilicon oxide layer 72, an Si layer 73, a Ge layer 74, a relaxed SiGelayer 75, a strained Si layer 76, a silicon oxide film 77, and a siliconnitride film 78. The Si layer 73 is formed on the silicon oxide layer 72and has a thickness of 50 nm. The Ge layer 74 is formed on the Si layer73 and has a thickness of 1 nm. The relaxed SiGe layer 75 is formed onthe Ge layer 74 and has a 30% Ge content and a thickness of 100 nm. Thestrained Si layer 76 has a thickness of 50 nm. The silicon oxide film 77has a thickness of 15 nm. The silicon nitride film 78 has a thickness of210 nm. In this structure, the Ge layer is provided and thus thedislocation density in the relaxed SiGe layer 75 can be reduced.

As the process step of forming a substrate 80, a SiGe layer (not shown)having a 30% Ge content is formed on an SOI substrate including asilicon substrate 71, a silicon oxide film 72 and an Si layer 73 with aGe layer 74 interposed between the SiGe layer and the SOI substrate.Then, by performing the same heat treatment as that shown in FIG. 14A,the SiGe layer is relaxed to form a relaxed SiGe layer 75. Other processsteps are the same as those described in FIG. 14A.

In this embodiment, the case in which an SOI substrate is formed bybonding has been described. However, such an SOI substrate as shown inFIGS. 14A, 15 and 16 may be formed by SIMOX.

Note that FIGS. 15 and 16 illustrates layer structures disclosed inJapanese Unexamined Patent Publication No. 9-180999.

(Other Embodiments)

In the foregoing embodiments, p-MOSFETs have been described as examples.However, an n-MOSFET may be used in the present invention.

Furthermore, the present invention is applicable to a heterojunctionbipolar transistor including a SiGe layer or a SiGeC layer. In such acase, Ge can be also evaporated.

In the foregoing embodiments, semiconductor devices in which a layercontaining Si and Ge serves as a hole channel have been described asexamples. However, the present invention is applicable to asemiconductor device including a layer containing Si and Ge not as achannel layer but as some other layer. In such a case, Ge can beevaporated by performing heat treatment in the process step of makingthe layer containing Si and Ge exposed.

Note that the present invention is not limited to use in the processstep described in the foregoing embodiments but is applicable to use inpretreatment for the process step of oxidizing a SiGe layer or a SiGeClayer. In such a case, Ge can be also evaporated.

1. A method for fabricating a semiconductor device, comprising of thesteps of: a) forming a compound semiconductor layer containing Si and Geabove a semiconductor substrate; b) removing part of the compoundsemiconductor layer to form a trench; c) performing heat treatment on asurface portion of the trench, and forming a Ge evaporated portion inpart of the surface portion of the trench at which the compoundsemiconductor layer is exposed; d) oxidizing at least part of thesurface portion of the trench to form a thermal oxide film; and e)filling the trench with an insulator to form a trench isolationincluding the thermal oxide film and the insulator.
 2. The method ofclaim 1, wherein the step c), the heat treatment is performed undervacuum.
 3. The method of claim 1, wherein the step c), the heattreatment is performed in a non-oxygen atmosphere.
 4. The method ofclaim 1, further comprising after the step a), the step of epitaxiallygrowing an Si layer on the compound semiconductor layer.
 5. The methodof claim 1, wherein the step a), the compound semiconductor layer isprovided on the semiconductor substrate with an insulating layerinterposed therebetween.
 6. The method of claim 1, wherein the heattreatment is performed at a temperature ranging from 700° C. to 1050° C.7. The method of claim 1, wherein the step d), part of the compoundsemiconductor layer extending inward from the surface thereof located atthe trench to a distance of 30 nm or less.
 8. The method of claim 1,wherein the step C), an upper portion of the compound semiconductorlayer is subjected to heat treatment, wherein in the step d), the upperportion of the compound semiconductor layer is oxidized to form a gateoxide film, and wherein the method further includes after the step d),the step of forming a gate electrode.